Click image for full scale version (will open in a new window)
Click image for full scale version (will open in a new window)
Patient: Male, 91 y/o. Admitted to the hospital after he fell in his home and was found after 24 hours on the floor. No complaints over chest pain. He is put under observation for commotio cerebri. Cerebral CT scan is negative. Known angina pectoris and has had one AMI in 1983 (vessel unknown). He has a prerenal kidney failure. Troponine level on admittance is 0.04. 8 hours later it is 0.03.
ECG description:
- Regulary irregular supraventricular rhythm
- Sinus rhythm
- Premature atrial contractions (PAC) in trigeminal pattern
- Normal cardiac axis
- Poor R wave progression
- Q wave in lead III
- 2mm horizontal ST-depression in leads V2-V4
ECG interpretation:
Sinus rhythm is interrupted by premature atrial contractions (PAC). Every second sinus beat is followed by a PAC, resulting in a trigeminal pattern. Each PAC resets the sinus node, and is therefore followed by a non-compensatory postextrasystolic pause, contributing to slowing the rhythm down. The ectopic P waves are best seen lead V1, where they are upright/positive. In the inferior leads (II, III, aVF), the same P waves are inverted. This indicates that the ectopic focus lies in or around the AV junction, depolarizing the atria in a retrograde fashion – away from the inferior leads, making the P wave negative. You will probably note that the first P wave in the ECG is negative in lead III. This could lead to confusion, as it would suggest that also this beat is ectopic. However, a diphasic or inverted sinus P wave in lead III is normal in small amounts of the population (Chou 2008:8). Also, note that the last P wave in the ECG has the same P wave axis and morphology as the rest of the ectopic P waves. Although it doesn’t seem to be much premature, it comes from the same ectopic pacemaker. It also breaks the trigeminal pattern.
Premature atrial contractions (PAC) and postextrasystolic pauses
Since we’re on the subject, let’s do a quick recap of the essentials regarding PACs and their pauses.
- A PAC originates from an ectopic site in the atria, and therefore produces a P wave morphology that differs from that of the sinus P wave. It will also produce a normal QRS, as the premature atrial impulse will conduct through His-Purkinje normally. Unless of course, some kind of intraventricular conduction delay is present or if aberrant conduction of the impulse occurs. A PAC produces a pause in the heart rhythm, which can be seen on the ECG. A PAC will usually depolarize the whole atrium (logically, since there is nothing there to stop the impulse from spreading all over the place). When the impulse therefore reaches the sinus node, the node will be “reset” and start a “new” sinus rhythm. This will be manifested on the ECG as a pause, which will be longer than the sinus cycle.
- Such a pause is either labeled a compensatory or a non-compensatory postextrasystolic pause. To differentiate these pauses, we need to look at the three intervals: the normal interval, the coupling interval and the postextrasystolic pause. The normal interval is the basic cycle in the ECG, in this ECG it is the sinus cycle, the cycle starting with the third P wave and ending with the fourth P wave in this ECG. The coupling interval is the interval between the sinus P wave and the premature P wave. The postextrasystolic pause is the pause after the premature beat, consisting of the interval between the premature P wave and the sinus P wave following the pause.
- Now, if the coupling interval + postextrasystolic pause is less than twice of the normal interval, the pause is non-compensatory. If the two added intervals equals twice or more than twice the length of the normal interval, the pause is compensatory. With PACs, non-compensatory pauses are the common finding. Only very rarely, a premature beat originating near or in the AV junction can spread only in the anterograde direction and avoid resetting the sinus node and thereby produce a compensatory pause.
- Now, let’s take a look at lead III from this ECG. By using a ruler or a caliper we will find that the coupling interval + the postextrasystolic pause is less than twice the length of the normal interval. Which is what we expected to find. This is a premature atrial contraction producing a non-compensatory pause.
Click image for full scale version (will open in a new window)
May 20, 2009
Click to see full scale version (will open in a new window)
Click to see full scale version (will open in a new window)
ECG description:
- Sinus rhythm, regularly irregular, at approximately 90 bpm
- PR interval normal and constant at 140ms
- Multiple premature atrial complexes
- Atrial trigeminy pattern with compensatory postextrasystolic pauses
- Axis normal at approx. 60°
ECG conclusion: Atrial Trigeminy with Compensatory Postextrasystolic Pauses.
Recognition of Premature Atrial Complexes (PAC)
Every second beat is followed by a premature atrial complex (PAC). A PAC is recognized by a premature QRS complex preceded by a premature P wave. As the pacing comes from the atria, the QRS complex is narrow and has the same morphology as the normal beats. PACs often gets mistaken for junctional premature beats, as the premature P wave easily gets hidden in the preceding T wave. Whenever presented with premature beats, always scan the T waves for hidden P waves. With completely hidden P waves, the giveaway is often a too tall T, which is taller than the other T waves in the same lead. Premature atrial impulses arise from irritable automaticity foci in the heart, and can be induced by several factors, as adrenaline (epinephrine) released by adrenal glands, increased sympathetic stimulation, precense of caffeine, amphetamines, cocaine or other β1 receptor stimulants. Also, digitalis toxicity can lead to PACs. When an automaticity focus in the atria becomes irritable due to one or several of the factors mentioned, it may spontaneously fire an impulse. Such a premature impulse will depolarize the surrounding myocardial tissue, and will show on the ECG as a premature beat. As irritable foci in the atria will lead to atrial contraction, a p wave will preceed the premature QRS complex. Since the impulse starts in the atria, it usually gets conducted normally through to the ventricles via the AV node, which shows on the ECG as a normal PR interval (unless underlying AV block of any kind) and a QRS complex that is similar to the other sinus beats on the ECG.
Different intervals during sinus arrhythmia
A premature beat is premature because it occurs earlier than expected. To be able to tell if a beat is premature, one needs to look at the preceding beats, comparing the normal beat intervals to the interval preceding the premature complex. If the RR or PP interval preceding an early beat is shorter than the normal RR or PP intervals, the beat is considered premature. There are three types of intervals during arrhythmias, that are important to understand the underlying mechanism when dealing with premature beats. These may be named differently in the books and literature, but the concept is the same:
Normal interval (N_int): The interval (PP or RR), ususally in milliseconds or millimetres between the beats in the underlying rhythm.
Coupling interval(C_int): The interval (PP or RR) between a normal sinus beat and the following premature beat.
Postextrasystolic pause (P_ex): The following pause after a premature beat. A postextrasystolic pause can be compensatory (where the SA node is not reset) and non-compensatory (where the SA node is reset).
The Postextrasystolic Pause: Difference between compensatory and non-compensatory
The cycle pause after a PVC is called a post-extrasystolic pauses. Such pauses are divided into two kinds: Compensatory Post-Extrasystolic Pauses and Non-Compensatory Post-Extrasystolic Pauses. These names are often too long to use, so the terms compensatory pause and non-compensatory pause are used instead.
Compensatory Post-Extrasystolic Pause
A PVC starts in the ventricles from an irritable, often hypoxic focus. Therefore, it only depolarizes the ventricles, not the SA Node. Therefore, the SA Node is not reset. So the SA Node fires as planned and on schedule. Often, if you use your caliper and measure PP intervals, you can spot that timely P within a PVC. The problem is though, that when the sinus node fires, the ventricles are still refractory, and the sinus impulse doesn´t get conducted. When this impulse reaches the ventricles, they´re not ready, and can´t depolarize. So there is a pause after the PVC as the ventricles finish repolarizing, making them receptive to the next sinus generated cycle. Remember that since the depolarization begins in the ventricular tissue, the SA Node will never know anything about this premature impulse. And if it doesn´t get depolarized by the impulse, it will not reset and will keep on pacing. And if the SA Node is not reset, the compensatory pause will be an exact multiple of the regular PP interval. So by measuring PP intervals, you can check if the pause is compensatory or not.
Non-Compensatory Post-Extrasystolic Pause
With non-compensatory pauses, the SA Node is reset and starts a new sinus cycle. The non-compensatory pause is not an exact multiple of the regular PP interval. The SA Node is usually reset by Premature Atrial Contractions (PACs) or Premature Junctional Contractions (PJCs). PVCs are sometimes followed by a non-compensatory pause, but only very rarely. Remember, for the SA Node to be depolarized by a PVC, there will have to be retrograde conduction through the AV Node. This is not very usual, but can happen. The take-home advice here, is that with non-compensatory pauses, you are usually dealing with a PAC or a PJC.
The Postextrasystolic Pause: Distinguishing compensatory pauses from non-compensatory pauses
By measuring and comparing intervals, and by setting up two simple formulas, we can check if a postextrasystolic pause is compensatory or not.
Compensatory: The pause is so long , that the distance between the two normal beats that surround the premature beat, is twice as long the normal interval in the underlying rhythm.
Non-compensatory: The pause is longer than the normal interval in the underlying rhythm, but not long enough for the coupling interval and the compensatory pause to double the length of the normal interval.
Click to see full scale version (will open in a new window)
As we can see here, the normal interval is 31mm. The sum of the coupling interval and the postextrasystolic pause is 62mm. The pause is compensatory, which means the SA node has not been reset. It continues to pace, undisturbed and unknowingly of the premature beat that just occured. By positioning a caliper on the normal interval and moving this distance two times to the right, you will land exactly on the P wave that follows the postextrasystolic pause. Remember that if the SA node had been reset, it would start a new cycle, and therefore the postextrasystolic pause would be shorter.
December 10, 2008